There is a silent killer taking over Sydney’s harbour. Jessica Hynes writes about the spread of Caulerpa taxifolia and how it affects biodiversity in our harbour.

Dave Thomas fears an invasion is upon us. He isn’t talking about the type of dooms-day, war-like invasion Australians feared would happen with the Japanese in WWII. But like the soldiers who sat and waited, watching for Japanese submarines entering our beloved harbour, Thomas looks out over Sydney, anxiously awaiting his enemy: the toxic marine alga Caulerpa taxifolia.

Caulerpa Taxifolia is spreading across Sydney's harbour. Image: Wiki Commons

The battle began on the coast of Monaco in 1984 where a small plot was discovered in the Mediterranean Sea. Because Caulerpa is a tropical species, it was thought the aquarium strain alga would not survive the cool waters of the Mediterranean. But it did.

Caulerpa has now conquered thousands of hectares of the world’s seafloor, including 14 waterways in New South Wales.

The alga is native to tropical Queensland, but the same cannot be said for NSW. It was first discovered in NSW waters outside its natural range, at Port Hacking in April 2000.

There is no definitive answer as to how the alga made its way into our waters, although scientists deem human intervention, the dumping of aquaria containing Caulerpa, the most likely reason.

Once at Port Hacking, discoveries were soon made at Lake Conjola, Pittwater and Botany Bay. And with a rapid reproductive process of fragmentation accelerated by anchoring boats that cut up the weed and relocate it to other areas, more populations quickly appeared up and down the NSW coast.

Since 2002, Industry & Investment NSW (I&I) has undertaken a continuous research and control program involving the treatment of over six hectares of Caulerpa with 1500 tonnes of salt in hope of its eradication. Yet 14 of the state’s estuaries remain infected.

According to marine ecologist Dr Paul Gribben, salt has been able to permanently eradicate an entire population of Caulerpa at Lake Macquarie but it has not yet been achieved elsewhere.

Dr Gribben, a research fellow at the University of Technology, Sydney (UTS), said: “There have been no wholesale efforts to remove Caulerpa from very large areas. It just can’t be done. There’s just too much of it. Too much Caulerpa and it costs too much money.

“The responsibility falls with the Industry & Investment department but they can only do what they have the funds to do. They are concerned but because they lack government funding they are restricted.”

Because of its aggressive reproduction process, Gribben argues there is no reason why it will not continue spreading.

Thomas is concerned that, because of its ability to grow rapidly, Caulerpa could eventually out-compete the luscious populations of native sea grass that occupy Manly’s Cabbage Tree Bay.

He said: “The sea grasses act as nursery grounds for many species. They are places where little juvenile fish can recruit, seek refuge from predation and have a source of food.

“Caulerpa will take over the areas populated by sea grass and this will impact on all aspects of animal life up and down the chain.”

Gribben, who has collaborated with I&I NSW on many research projects, believes the spread of Caulerpa is enhanced by deteriorating beds of sea grass and the situation is not a case of one species ‘out-competing’ the other.

“The sea grass is stressed by urbanisation, nutrients and sewerage,” he said.

“They get gaps in them or they start retreating and that opens up space for Caulerpa to come in. That doesn’t mean Caulerpa is out-competing sea grasses, it just means that it is taking advantage of the space.”

World-wide studies, including those held by Gribben at UTS, show that the composition of fish communities and invertebrates change when Caulerpa is introduced, notably because of its production of toxic substances that deter many species.

Dr Gribben said: “Fish don’t like to forage in it, they don’t like to eat it and they don’t like to go near it. What we may eventually see is a loss of herbivorous fish because they will move away.”

In Manly Cove and North Harbour, years of heavy anchoring activity has wiped out a lot of the native sea grass, which has made it particularly vulnerable to Caulerpa.

Manly Harbour Image: Wiki Commons

With sites including Little Manly Beach, Clontarf, Forty Baskets and Quarantine Beach infested, Cabbage Tree Bay is one of the only areas in the Manly region where there is currently no Caulerpa.

Thomas believes the weed would be catastrophic and provoke precious invertebrates and species of rays or octopuses to disappear.

He said: “It [Caulerpa] won’t support the number of species that sea grass will so you’ll loose fish species and this will impact diving and snorkeling, which are significant to the tourist activity and culture of Manly.”

According to I&I NSW, permanent eradication of Caulerpa in NSW waters is unfeasible due to the large size of existing Caulerpa beds. And that’s why its focus is on smaller populations.

But Thomas believes that I&I needs to be more assertive in protecting valuable areas that are at high risk of becoming infested, such as Cabbage Tree Bay.

He said: “There has to be a proactive way of coming in and enforcing rules. A zero tolerance ‘no anchoring’ policy would be appropriate but I think they’re just too scared to do it.”

Fishing has been illegal in the bay since it was declared an aquatic reserve in 2002 but boats have been able to continue anchoring, causing continuous damage to the sea grass.

Thomas said: “The anchors just rip at the sea grass. The chain mows the bottom, destroying anything it its path, and a lot of the boaties don’t realise what they’re doing. There are no visible signs around.”

“It only takes one fragment and by the time the weed becomes obvious, it will be too late to do anything,” he said. “It will be rife just like it is everywhere else.”

Environmental activists like Thomas and Manly Greens Councillor Cathy Griffin are in support of moves by the NSW Parks and Wildlife Service, who control the reserve, to ban boats from the bay entirely.

But they face fierce competition from the boating community who believe their right to enjoy public space will be infringed.

Michael Chapman, President of the Boat Owner’s Association of NSW, said: “We recognise that the sea grass is precious and that Caulerpa is a threat to Cabbage Tree Bay but with the installation of some sea grass friendly public moorings and clear signage boats should be able o co-exist with the environment.”

“Boats have been anchoring here for years, it’s a safe-haven from southerly winds, and there’s still no Caulerpa. Completely banning boats would be way too extreme.”

In 2002, Caulerpa was also discovered in South Australia’s Port River and its state government imposed an anchoring ban on all vessels.

Although Caulerpa is still present in the river, the ban was lifted in March 2011 and is now classed as a ‘containment area’, with Biosecurity SA pleading boat users to continue being vigilant to ensure the weed does not spread beyond these new boundaries.

This raises the question: Are anchoring bans effective?

One Adelaide fisherman commented on an online fishing forum Strike & Hook: “The no anchoring thing wasn’t taken seriously at all and never policed. The weed is still in the system so figure that one out. Bloody waste of time and many people missed out on fishing in there that whole time.”

Gribben argues that sea grasses, which are damaged by anchoring, are vital to the prevention of Caulerpa invasion so a ‘no anchoring’ policy in a site like Cabbage Tree Bay would be valuable.

He said: “Most of the research shows that if sea grasses are healthy Caulerpa can’t get a foothold and for this reason it’s essential we care for areas where sea grass exists.”

But it’s not just the sea grasses that need protecting, said Gribben, it’s the bare sand too.

“They have really high biodiversity of bivalves, clams, mussels and worms and, although they don’t sound exciting, they’re really important to how our estuaries function and their health.”

According to Gribben, Caulerpa is an ecosystem engineer and while other invasive species eat and compete with other organisms, Caulerpa transforms and engineers the environment.

He said: “It’s the environmental change that affects other species, it’s not Caulerpa per se. Caulerpa affects the environment and the environment affects the animals.

“The Caulerpa sucks all the oxygen out of the sediment, makes the habitat toxic and most of the animals that cannot adapt to that environment will die. It’s likely we will see localised extinctions of some species.””

Caulerpa is also rife in the waters of Pittwater, with beds at The Basin, Bayview, the western side of the Barrenjoey Peninsula and surrounding Scotland Island.

It is one of only two Caulerpa affected estuaries where commercial net fishing is still allowed.

Most of the areas in Pittwater that were commercially hauled in the past have a Section 8 Caulerpa netting closure but other Caulerpa infested areas in the region remain open to commercial net fishing.

At the present, I&I NSW is considering further closures in Pittwater to protect beds of native sea grass and, if these are implemented, the Caulerpa closure in Pittwater will be revoked.

Dr Jonathan King, Greens candidate for Pittwater, believes this would be a viable option for Pittwater.

“Given that there is a clear lack of funding that makes the prospect of eradicating Caulerpa in our area improbable, the idea of closing off areas where native sea grass exists would be effective in securing their protection.

“Although a removal program would be ideal, this would be affordable for the state government and certainly cost-efficient.”

The proposal at Pittwater would be suitable for Cabbage Tree Bay, Thomas said.

“If it can be done there, if sea grass beds could be closed to anchors and boats, then why can’t it be done here? All we need is a ‘no anchoring’ zone really!

“This really is a battle worth winning. And while it may be too late in some places, there is still a chance [for] victory here.”

Whatever it is, King is adamant something needs to be done.

“Caulerpa beds are as bad as cane toads. They’re like submarine cane toads.”

And no one likes submarines in our harbour.

This article was first published on Reportage-Enviro by Jessica Hynes.

Reportage is the Australian Centre for Independent Journalism’s (ACIJ) web magazine. It is dedicated to high quality independent journalism.

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